The present invention relates to a reference voltage generator and, more particularly, to such a generator of a band-gap regulator type used in a CMOS transistor circuit.
Although various types of reference voltage generators are employed in a transistor circuit to generate a reference voltage, the so-called band-gap regulator is advantageous in generating a reference voltage having characteristics stabled against change in temperature and in power supply voltage. The band-gap regulator requires a pair of bipolar transistors operating in different current densities from each other. The band-gap regulator used in a CMOS transistor circuit also has a pair of bipolar transistors, accordingly.
Referring to FIG. 1, the band-gap regulator 100 as a reference voltage generator used in the CMOS transistor circuit has a pair of bipolar transistors 4 and 5 and an operational amplifier 14 constituted of CMOS transistors. The collectors of the transistors 4 and 5 are connected to a power supply line 18. The emitter of the transistor 4 is connected through a resistor 1 to a ground line and further to the inverting input terminal 6 of the amplifier 14. The emitter of the transistor 5 is connected through resistors 2 and 3 to the ground line. The node of the resistors 2 and 3 is connected to the non-inverting terminal 7 of the amplifier 14 which has an output terminal lead as a reference voltage output terminal 15. The terminal 15 is connected through resistors 16 and 17 to the ground line, and the node of the resistors 16 and 17 is connected to the bases of the transistors 4 and 5.
Since the emitter of the transistor 4 is connected to the ground line through one resistor and the emitter of the transistor 5 is done through two resistors, the vase-emitter voltages of the transistors 4 and 5 are different from each other. That is, the transistors 4 and 5 operate in the different current densities. The difference in base-emitter voltage DVBE between the transistors 4 and 5 is therefore represented by the following equation (1): ##EQU1## wherein VBE4 and VBE5 are the base-emitter voltages of the transistors 4 and 5, R1 and R3 are the resistance values of the resistors and n is the ratio in emitter are of the transistor 5 to the transistor 4. Further, k represents Boltzmann constant, T does absolute temperature and q does electron charge.
The current I5 indicative of the following equation (2) thus flows through the transistor 5: ##EQU2## wherein R2 is the resistance value of the resistor 2. Assuming that the current I4 flows through the transistor 4, the voltage Va at the node 6 is represented as follows: ##EQU3##
On the other hand, the base voltage Vb of the transistors 4 and 5 are as follows: EQU Vb=Va+VBE4={R17/(R16+R17}.multidot.Vo
wherein R16 and R17 are the resistance values of the resistors 16 and 17 and Vo is a reference voltage at the output terminal 15. From the equations (3) and (4), the reference voltage Vo is derived as follows: EQU Vo={(R16+R17)/R17}.times.{VBE4+(R3/R2).multidot.(kT/q)ln(n.multidot.R3/R1)} (4)
Thus, the output voltage Vo is dependent on the ratio in resistance value of between the resistors 16 and 17 and the voltage Va at the node 6 indicative of the equation (3). The voltage Va is in turn dependent on the ratio of the resistors R3 to R2, the emitter area ratio n, and the ratio of the resistors R3 to R1.
The ratio of the resistors R3 to R2 is, however, cannot be made large because the input offset voltage of the amplifier 14 is multiplied by that ratio. The emitter ration n is required to made small in order to reduce the area occupied by the transistors 4 and 5. The ratio of the resistors R3 to R1 is also required to made small because the voltage drop across the resistor R3 is to be small for the purpose of attaining the transistor operation for the transistors 4 and 5. As a result, the voltage Va becomes low inevitably. For example, such designs are made that R1=1 k.OMEGA., R2=14 k.OMEGA., R3=65 K.OMEGA., and n=10, the voltage Va takes a value of 0.05 V.
Such a low voltage Va causes the MOS transistors in the operational amplifier 14 operate in a non-saturated region. Consequently, the output voltage of the amplifier 14, i.e. the reference voltage Vo, is easy to subjected to the noise voltage of the power supply voltage. In other words, the reference voltage Vo is varied in accordance with the noise components of the power supply voltage.